The invention relates to radio receivers having the capability of receiving digital television (DTV) signals, such as digital high-definition television (HDTV) signals.
The first detector in a television signal receiver converts radio-frequency (RF) signal from a selected one of the television broadcast channels, which channel occupies one of specific 6-MHz-wide portions of the electromagnetic wave frequency spectrum, to an intermediate-frequency (IF) signal in one particular 6-MHz-wide portion of that spectrum above or below that in which television broadcast channels are assigned. The first detector comprises a first local oscillator for generating first local oscillations, a first mixer for mixing the selected one of the television broadcast channels with the first local oscillations to generate the IF signal and its image, and a frequency-selective filter for passing the IF signal while blocking the image. The conversion is typically carried out by superheterodyning the RF signals, which is to say mixing the RF signals with first local oscillations of a frequency substantially higher than the frequencies in the television channel of highest frequency. The first detector is used to convert a selected RF signal to an IF signal in order that up to 60 dB or more amplification can be done in that particular 6-MHz-wide portion of that spectrum using intermediate-frequency amplifiers that do not require adjustable tuning. Amplification of the received signals is necessary to raise them to power levels required for further signal detection operations, such as video detection and sound detection in the case of analog TV signals, and such as symbol decoding in the case of digital TV signals. The first detector usually includes variable tuning elements in the form of pre-selection filter circuitry for the RF signals to select one among the various 6-MHz-wide television channels and in the further form of elements for determining the frequency of the local oscillations used for super-heterodyning the RF signals. The pre-selection filter circuitry for the RF signals usually includes a radio-frequency amplifier for strengthening the signals supplied to the first mixer. In TV receivers of more recent design the first local oscillator signals are often generated using a frequency synthesizer, in which the first local oscillator signals are generated with frequency regulated in adjustable ratio with the fixed frequency of a standard oscillator.
The mixers and IF amplifiers in a digital television receiver have different design requirements than the mixers and IF amplifiers in an analog television receiver. The mixers and IF amplifiers in an analog television receiver are designed not to exhibit an overall gain response that is linear. Symbol decoding in a DTV receiver involves a procedure known as data-slicing, which determines which of a number of amplitude bins the amplitude of the baseband DTV signal currently resides in, each of which amplitude bins is associated with a particular symbol decoding result expressed as a group of successive bits of data. In order that data-slicing be carried out optimally with the current DTV broadcast standards, the mixers and IF amplifiers in a DTV receiver must exhibit an overall gain response that is linear. Where mixing is done by multiplying selected DTV signal with sinusoidal local oscillations, spectral purity of the oscillations (i. e., freedom from harmonic distortion) is important, and the mixers should be linear multipliers. The gain provided by the IF amplifiers should be very linear.
Television signal receivers for receiving digital television (DTV) signals that have been proposed by the Grand Alliance, a group of DTV proponents including Zenith Electronics Corporation, use plural-conversion radio receivers. During the first detection procedure in these plural-conversion radio receivers, DTV signal in a selected one of the ultra-high-frequency (UHF) channels is up-converted in frequency to first intermediate-frequency signal in a first intermediate-frequency band centered at 920 MHz. This puts the image frequencies above 1 GHz, making them easy to reject by fixed-tuned front-end filtering. The upconverted DTV signals are then amplified in a first intermediate-frequency amplifier that uses ceramic resonators for tuning. The resulting amplified first intermediate-frequency signal is then down-converted in frequency by mixing it with 876 MHz local oscillations, resulting in a second intermediate-frequency signal in a second intermediate-frequency band 6 MHz wide centered at 44 MHz. The overall amplitude and phase characteristics of the receiver are controlled using a surface-acoustic-wave (SAW) filter for selecting the second intermediate-frequency band. This second intermediate-frequency signal, as selected by the SAW filter, is then amplified in a second intermediate-frequency amplifier. The response of the second IF amplifier is then synchrodyned to baseband. This synchrodyning procedure can be a direct one in which the response of the second IF amplifier is synchronously detected at the frequency of the data carrier in the second IF band. Alternatively, this synchrodyning procedure can proceed by stages, with the response of the second IF amplifier being first down converted to a third and final intermediate-frequency band and then synchronously detected at the frequency of the data carrier in the final IF band. This alternative synchrodyning procedure is preferred where synchronous detection is to be done in the digital regime, rather than the analog regime, since the sampling rates required in analog-to-digital conversion can be lowered sufficiently to make such conversion practical with currently available technology.
Radio receivers for receiving digital television signals, in which receiver the final intermediate-frequency signal is somewhere in the 1-8 MHz frequency range rather than at baseband and is digitized before synchrodyning to baseband, are described by C. B. Patel et alii in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d. The entire specification and drawing of U.S. Pat. No. 5,479,449 is incorporated herewithin by reference, particularly FIGS. 2-5 and the specification descriptive of various ways to implement bandpass phase tracking for a vestigial-sideband signal. The use of infinite-impulse response filters for developing complex digital carriers in such receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,548,617 issued Aug. 20, 1996 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER USING RADER FILTERS, AS FOR USE IN AN HDTV RECEIVERxe2x80x9d. The design of receivers for both VSB and QAM signals in which both types of signal are processed through the same intermediate-frequency amplifiers receivers is described by C. B. Patel et alii in U.S. Pat. No. 5,506,636 issued Apr. 9, 1996 and entitled xe2x80x9cHDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTIONxe2x80x9d. U.S. Pat. No. 5,606,579 issued Feb. 25, 1997 to C. B. Patel et alii and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH FINAL I-F CARRIER AT SUBMULTIPLE OF SYMBOL RATE, AS FOR HDTV RECEIVERxe2x80x9d further explains bandpass trackers. These patents and patent applications are all assigned to Samsung Electronics Co., Ltd., pursuant to employee invention agreements already in force at the time the inventions disclosed in these patents and patent applications were made.
The present invention concerns solutions to problems encountered in the design of the intermediate-frequency amplification portions of a digital TV receiver which supply the final intermediate-frequency signal somewhere in the 1-8 MHz frequency range.
In a digital signal receiver there is great concern in carefully controlling the overall amplitude and phase characteristics of the receiver in order to minimize intersymbol error, while at the same time rejecting interference from signals in adjacent channels. Getting flat amplitude response within xc2x11 dB over a bandwidth of 5.5 to 6 MHz, while maintaining acceptable group delay characteristics, requires SAW filtering with a great number of poles and zeroes to define the receiver bandwidth. It is difficult and expensive to implement such SAW filtering for a very-high-frequency (VHF) band, such as 41-47 MHz. Also, the insertion loss is quite high in a VHF band, typically 15-17 dB for the 41-47 MHz band. The SAW filtering to define receiver bandwidth can be more easily implemented for an ultra-high-frequency (UHF) band, such as at 917-923 MHz, the inventors observe, as long as care is taken to drive the SAW filter from the optimal source impedance specified by its manufacturer. This is because the xcex94f/f ratio of 6 MHz to 920 MHz is substantially lower than the xcex94f/f ratio of 6 MHz to 44 MHz. Insertion losses also tend to be lower in a UHF band, typically 10-12 dB for the 917-923 MHz band. SAW filters for the VHF band are commonly on lithium niobate substrates; but SAW filters for the UHF band are constructed on other substrate material such as gallium arsenide.
Analog TV receivers receive modulation, the higher-energy portions of which are descriptive of synchronizing pulses, and the lower-energy portions of which are descriptive of gamma-corrected luminance signal and a chroma subcarrier. Automatic gain control in a digital TV receiver is designed primarily to achieve as low noise figure as possible. There is less concern with regard to the linearity of intermediate-frequency amplification, particularly if the final intermediate-frequency signal can be band-pass filtered before video detection takes place, so harmonic distortion of the final intermediate-frequency signal can be selected against. In a digital TV receiver while low noise figure is desirable, small amounts of random noise are strongly rejected by quantizing effects in the data-slicing and trellis decoding associated with symbol decoding. Linearity of intermediate-frequency amplification is extremely important, particularly as automatic gain control is applied. This is so that data-slicing operations during symbol decoding are less susceptible to error in deciding the symbol codes described by the modulation. Linearity of intermediate-frequency amplification generally is better maintained using reverse automatic gain control methods than by using forward automatic gain control methods which are favored in analog TV because they are less likely to increase noise figure. In forward automatic gain control of a transistor amplifier stage, gain is reduced by biasing the stage into a partial clamping condition by increasing the direct current through the transistor. In reverse automatic gain control of a transistor amplifier stage, gain is reduced by reducing the signal current the amplifier transistor supplies to its load, by decreasing the effective transconductance of the transistor. The effective transconductance of the transistor can be reduced, for example, by biasing the transistor to decrease the direct current through the transistor.
Automatic gain control methods are generally more easily practiced at lower intermediate frequencies, where capacitive effects are less pronounced, than in the 920 MHz first-intermediate-frequency band. The use of fixed gain in the first IF amplifier simplifies driving the SAW filter from the optimal source impedance specified by its manufacturer. The use of fixed gain in the first IF amplifier facilitates those portions of the receiver up to and including the second mixer being utilized both for DTV reception and for analog TV reception during the transition era to DTV broadcasting when analog TV broadcasting continues. The second IF amplifiers for DTV reception and for analog TV reception can then be separate, with suitable respective IF automatic gain control for each mode of reception. If an initial radio-frequency (RF) amplifier is employed, its automatic gain control is delayed from that used for automatic gain control of the IF amplifiers, the RF amplifier AGC being used primarily to prevent front-end overload during strong signal reception. Accordingly, the delayed AGC of the RF amplifier can employ reverse AGC, with the loss of noise figure occurring only during the reception of strong signals where signal-to-noise ratio is likely to be adequate in any case.
When the final intermediate frequency is close to baseband as compared to its bandwidth, the harmonic distortion of the lower frequencies in the final IF signal falls in the same portion of the frequency spectrum as the higher frequencies in the final IF signal. Accordingly, bandpass or lowpass filtering of the final IF signal will not be very effective in suppressing harmonic distortion. It is preferable, then, to avoid amplification of the third intermediate frequencies that might be non-linear in nature.
Amplifier stages with automatic gain control are accordingly better located in the second intermediate-frequency amplifier than in the first intermediate-frequency amplifier or in a third intermediate-frequency amplifier. It is also easier to get broadband gain with less power at frequencies in the second IF band than those in the first IF band, since it is easier to overcome stray capacitance shunting resistive collector or drain loads for the amplifier transistors.
The invention is embodied in receiving apparatus for a selected one of a plurality of digital television signals transmitted in ones of the 6-MHz-wide channels of the electromagnetic frequency spectrum that are used for television broadcasting, which receiving apparatus digitizes a final intermediate frequency signal supplied from a triple-conversion radio receiver and synchrodynes the digitized final intermediate frequency signal to baseband. The triple-conversion radio receiver selects one of the plurality of digital television signals for upconversion to that portion of the electromagnetic frequency spectrum above the channels that are used for television broadcasting, amplifies a first intermediate-frequency signal in a first intermediate frequency band generated by that upconversion, amplifies a second intermediate-frequency signal in a second intermediate frequency band generated by downconverting the amplified first intermediate-frequency signal, and supplies a third intermediate-frequency signal in a third intermediate frequency band generated by downconverting the amplified second intermediate-frequency signal. This third intermediate-frequency signal is the final intermediate-frequency signal supplied to the analog-to-digital converter for being linearly converted to a digitized third intermediate-frequency signal. Digital synchrodyning circuitry synchrodynes the digitized third intermediate-frequency signal to baseband and thereby generates at least a real component of digital baseband signal. Circuitry responsive to at least the real component of the digital baseband signal recovers a stream of digital data descriptive of the video and audio portions of television programming.
More particularly, the triple-conversion radio receiver comprises a first local oscillator for generating first local oscillations of a frequency adjustable over a band of frequencies located below the first intermediate-frequency band; a linear first mixer for generating the first intermediate-frequency signal by heterodyning the first local oscillations and the selected digital television signal together, then suppressing the image of the first intermediate-frequency signal; a first intermediate-frequency amplifier for amplifying the first intermediate-frequency signal to generate the amplified first intermediate-frequency signal; a second local oscillator for generating second local oscillations; a linear second mixer for generating the second intermediate-frequency signal by heterodyning the second local oscillations and the amplified first intermediate-frequency signal together, then suppressing the image of the second intermediate-frequency signal; a second intermediate-frequency amplifier for amplifying the second intermediate-frequency signal to generate the amplified second intermediate-frequency signal; a third local oscillator for generating third local oscillations; and a linear third mixer for generating the third intermediate-frequency signal by heterodyning the third local oscillations and the amplified second intermediate-frequency signal together, then suppressing the image of the third intermediate-frequency signal. The image suppression filter also suppresses response to harmonic distortion that may have arisen when automatically controlling the gain of the second intermediate-frequency amplifier. The third mixer is connected for applying the third intermediate-frequency signal to the analog-to-digital converter without any substantial loss of linearity. Accordingly, the problem of harmonic distortion in the final intermediate-frequency signal supplied to the analog-to-digital converter is avoided.
In a further aspect of the invention the first intermediate-frequency amplifier comprises a first surface-acoustic-wave filter preceded in cascade connection by a buffer amplifier for driving the first surface-acoustic-wave filter from a prescribed source impedance and providing amplification to overcome the insertion loss of the first surface-acoustic-wave filter. A flat-amplitude response with sharp cut-off of response to reject adjacent channel signals is more readily achieved in the first intermediate-frequency band above the bands assigned for television broadcasting than in the second intermediate-frequency band below the bands assigned for television broadcasting.
In a further aspect of the invention the second intermediate-frequency amplifier comprises a plurality of amplifier stages, at least one of which is provided with reverse automatic gain control. The application of reverse automatic gain control so as to maintain linearity of amplification is easier to do in the second intermediate-frequency band below the bands assigned for television broadcasting than in the first intermediate-frequency band above the bands assigned for television broadcasting.
In receivers designed for receiving over-the-air broadcasting, provision should be made for avoiding overload of the first mixer. Accordingly, a radio-frequency amplifier is provided with reverse automatic gain control delayed with respect to the gain control of amplifier stages in the second intermediate-frequency amplifier.